Corrected Gamma Spectra (CGS)

Raw spectra cannot be compared or summed because differences in the
spacecraft's temperature, voltages, or other factors may cause
variations in the energy scale from spectra to spectra.
Using gain, offset, and linearity engineering information
from the Odyssey spacecraft and associated correcting algorithms,
the raw gamma spectra are corrected to the
same energy scale, which allows direct comparisons of individual
spectra and the summing of multiple spectra. The Mars Odyssey Gamma
Ray Spectrometer Corrected Gamma Spectra
(CGS) data set is a time series collection of these corrected gamma
spectra measurements from the Gamma Sensor Head.

Corrected Gamma Spectra data are composed of corrected gamma
spectra and the associated timing, spatial and engineering
information. The CGS consists of the cumulative counts of
gamma rays at the detector over one collection interval binned
into 16384 channels based on energy. The collection interval
is approximately 19.7 seconds, but may vary over the course of
mapping. The counts in Channel 0 are all the counts that would
have been shifted to 0 or less than 0 by the spectrum shifting
algorithm. The counts in Channel 16383 are all the counts with
nergies greater than 10MeV, and counts in Channel 16382 are
all the counts that belong in either 16382 or would have been
shifted to 16383. The timing and spatial data provided with
the spectra includes spacecraft clock values and spacecraft
geometry data. The sc_ev_time, utc time and spatial fields are
all recorded at the center of the collection interval.

The CGS is intended to be the first intermediate data product
available for the gamma portion of the GRS. These data should be
useful to those scientists who are experienced in gamma
spectroscopy.

Data Collection, Ingest, and Processing

The three Gamma-Ray Spectrometer instruments collects a new spectrum
(pixel) approximately every 20
seconds, 360 times per orbit. Approximately 4200 spectra are
received every day. The data (both science and
engineering) are downloaded from the spacecraft by the Jet
Propulsion Laboratory (JPL) into the Telemetry Data System
(TDS). The TDS sends data to a process that translates data
packets and examines instrument health via messages. Data are
output to a spooler that passes it to the University of Arizona
(UA) database ingest process. The ingest process inputs raw data
into the UA database. Gamma data are processed through a number
of programs to yield corrected gamma spectra.

Data Reprocessing

During the mission, data may be periodically reprocessed.
For more information on reprocessing of data, see the:

Both of the above documents are in the section of the
GRS Data Interface pages.

How to Use the GRS Data Interface, Step by Step

For new users, this web page provides help in a step-by-step way,
and will walk you through a typical session using the GRS Data Interface
to obtain Gamma-Ray Spectrometer data from Mars Odyssey.

When you first access the GRS data node, you'll notice that the web page
is divided into two vertical sections.

The area on the left-hand side of the
GRS Data Interface, as shown at left, is a selection "tree."
Entries titled "Introduction,"
"Calibrated Products," "Uncalibrated products," etc. are listed.
Note that the tree contains folders with a
symbol next to it. The symbol indicates
that there are items under that category that are not displayed. Clicking on a
symbol changes it to a
symbol and all the items under that
category will be displayed.

The area on the right-hand side of the GRS Data Interface changes, depending
on which part of the decision tree is currently active. It is used to
enter dates for database queries, provide informaton on downloaded file names,
or to display information.

Before You Download...

Before you execute a query using the GRS Data Interface, please do
the following to prepare:

Decide which type of data you wish to download. Remember that Corrected
Data (also called "Calibrated Data") is an intermediate product
that has been processed. Uncalibrated data, on the other hand,
is raw data that has had timing and spatial data added.
For more detailed information on the data products, see the
data product links on the left menu bar.

Consider your connection speed to the internet and your computer's system resources
such as disk space and RAM when deciding how many days of data to download per file.
It is best to choose time spans on the scale of a week or several weeks at
most for Corrected Gamma spectra, for example, due to the large output file
sizes. On any type of data it is best to choose one month or less time spans to
maximize the download time and prevent overloading your system's resources.

Decide on the beginning and ending dates and times for the data you wish to
download. Keep in mind the flight phases of the spacecraft, and
when the desired data was created in the spacecraft's
timeline.

Decide on the geographical area of interest, delimited by Martian
latitude and longitude, if applicable.

Once you have performed the above four steps, you are ready to begin downloading data.

Mouse clicking instructions are included for the PC-style, two-button mouse. (Left-click)
means to click on an icon or text with the left mouse button. Macintosh users have a one-button mouse
typically, and should click once with their mouse when (Left-click) is in the instructions.
UNIX users with three-button mouses should also use their left mouse button to (Left-click).

Using the GRS Data Node to Query The GRS PDS Database

Choose the category of data you wish to download, either Calibrated or Uncalibrated:
(Left-click) once on the data category to expand the selections.
For example, if you wished to choose a Calibrated Product, you would (Left-click) once on
. It would then show all the calibrated
products available and appear as:

After the selections are displayed and open, (Left-click) once on the data type desired. For example,
if you wished to download Corrected Gamma Spectra data, you would (Left-click) once on
. The type of data you have selected will
then be highlighted with the darker grey-purple:

In the right-hand side of the GRS Data Node window, a description of the chosen data type
will appear, including descriptions of all field (column) types in that particular data,
the units of each field (column), and other information.

If you would like to print the column-field information about this type of data,
(Right-Click) in the Column Descriptions area, and use your web browser's ability to
"Open this Frame in a New Window." You may then print from that new window. This
will give you a hard-copy description of all of the column descriptions, their units of
measure, etc. Return to the main GRS Data Node window before proceeding.

To enter the dates or other database query criteria, click on the
button to continue.

Note that data entry areas appear that allow you to constrain your database query by
time, or constrain it by spatial region (if available). Also, an option to exclude
data marked bad is offered.

Enter your latitude and longitude constraints. The Data Interface
allows you to Constrain by Spatial Region. Note the following area for
changing the latitude and longitude:

If you do not wish to select the entire planet, proceed to the next step.
If you wish to select the entire planet, go to
step 9.

Click on the link to
activate the Latitude-Longitude Chooser. The following new
window will appear:

Now you may enter the latitude and longitude in one of two ways:

By pointing and clicking. You will draw an imaginary box,
from the upper-left corner of the region you wish to specify,
to the lower-right corner. This is identical to drawing a box
in a drawing program on your computer.

Place the mouse cursor over the
MOLA image of Mars. Notice that the mouse cursor turns into
a crosshair ().

Locate the cursor
on the upper-left corner of the region box you wish to draw.

Click and hold the (Left-Mouse) button. Pull down and to the
right, until you reach the lower-right corner of your desired
rectangular area. The corresponding coordinates will appear in
the latitude and longitude data entry area below the MOLA map.

By entering the latitude and longitude manually in the data entry
area below the map:

Click in the area.
Type in the starting latitude of your region of interest, from
+90 (Martian North Pole) to -90 (Martian south pole).

Click on the area.
Type in the ending latitude of your region of interest.

Click on the area.
Type in the starting longitude in degrees East around Mars, from
0 degrees to 360 degrees.

Click on the area.
Type in the ending longitude of your region of interest.

In either case, note that you may not specify a region that
spans the 360-degree
longitude mark. As an example, you may not specify a region from
350 degrees to 20 degrees in longitude. This area of interest
would have to be split into two regions, one from 350 degrees to
360 degrees, and the other from 0 degrees to 20 degrees.

Click on the button
after the correct longitude and latitude of your region of
interest are entered.

Now you will enter the end date and
time (or second set of spatial constraints). If you wish to
collect 10 days of data from the start time entered above, you may enter "10" in the
range box: and press (Enter). The query
tool will automatically enter the correct end date.

OR

You may enter the end date and time under UTC Stop Time by
(Left-Clicking) as above and choosing the correct values. An example end time would be
May 4, 2002, at 00:00:00 GMT. The image below shows how your query tool would look after
entering these example values:

Decide whether to exclude bad data, or not. If you leave the box checked as shown:

Data marked as bad will NOT be included in your data set. If you wish to include bad data,
(Left-click) once on the box to un-check it. It is almost always recommended that bad data
be excluded.

(Left-click) on the button to start
compiling your query. Then an output like the following will appear:

If no data matching your selection criteria is found, you will get a message that says:
In those cases, you need to go back and modify
your query.

Note that in the example above the data is compiled but no data files are downloaded yet.
You still may go back and change your criteria. If you wished to modify this query, pressing the
button will allow you to go back and redo it.

All files are downloaded in zip (PC compressed) file format. The above gives you the number
of records that matched your query, and the estimated file size of your data file once it
has been unzipped.

To download the file, click on the link to it (on the estimated file size) under "Download."
In our example above, you would left-click on .

A status window, generated by your web browser, will then appear and inform you of the progress
of the download. Below is an example window from Netscape, version 7:

Click on the "Save to File" in your browser's download window. Then a window similar to the
following will appear, prompting you to choose a folder to download the file to:

Make certain you have adequate disk space on your computer before you save the file.

Click on the button (or equivalent, depending on your browser)
to save the file. Depending on the size of the file to be downloaded, this can take a few minutes or many minutes,
depending on your internet connection speed, your computer's speed, and the size of the file.

Use the Data Viewer software provided by the GRS PDS Data Node to quickly
peruse the data, or
write custom programs in the language of your choice to read and utilize the data.
A library of source code that will parse the uncalibrated (EDR) and calibrated (CGS) data product files
is included in the folder. This library allows a programmer to build
applications that display or manipulate the GRS data. This source is written in the Java language,
and requires version 1.4 of the Java Runtime Environment (JRE) or Java Software Development Kit (SDK).
For those who wish to use other languages, consult the Labels files to create structures to read in
the files for each type of data.